Process for the separation of styrene from ethylbenzene

ABSTRACT

STYRENE IS SEPARATED FROM ORGANIC MIXTURES COMPRISING STYRENE AND ETHYLBENZENE BY CONTACTING THE MIXTURE (FEED MIXTURE) AGAINST ONE SIDE OF A POLYURETHANE ELASTOMER MEMBRANE UNDER PERVAPORTAION PERMEATION CONDITIONS AND WITHDRAWING AT THE OTHER SIDE A VAPOROUS MIXTURE HAVING INCREASED STYRENE CONCENTRATION. THE POLYURETHANE ELASTOMER EMPLOYED CONTAINS POLYETHER OR POLYESTER GROUPINGS. EXAMPLES OF POLYURETHANE ELASTOMER POLYMERS ARE POLYMERS OF THE FORMULA   -(OOC-NH-R2-NH-COO-(A)N-OOC-NH-R2)M-NH-COO-R3-OOC-NH-   (R2-NH-COO-(A)N-OOC-NH-R2-NH-COO)M-   WHEREIN R, R1, R2 AND R3 ARE DIVALENT ORGANIC RADICALS, R4 IS HYDROGEN OR AN ORGANIC RADICAL, A IS   -R-OOC-R&#39;&#39;-CO-, OR -CH2-CH(-R4)-O-CH2-CH(-R4)-   N IS AN INTEGER IN THE RANGE OF 3-500 AND M IS NA INTEGER INDICATING THE DEGREE OF POLYMERIZATION.

' United States Patent once 3,776,970

Patented Dec. 4, 1973 membrane, said membrane containing a grouping selected 3,775,970 from the group consisting of polyester and polyether, and

PROCESS FOR THE SEPARATION OF STYRENE withdrawing from the second side of the membrane a FROM ETHYLBENZENE vaporous mixture having a higher concentration of styrene William F. Strazik Wilbraham Mass., and Eh Perry, 1 th 1 Stomer St. Louis, Mo., assiguors to lVIonsanto Company, St. 5 than the aforesaid mlxture' Po yure am e a L is M 0 branes employed in the process of the present invention Nis rirawiir Filed Dec. 14 1972 Ser. No. 315,266 are highly efiicient in Separating Styrene P ethyl- Ch c07 7 0'2 benzene using pervaporation separation techniques. The US. Cl. 260-669 A 6 Claims present invention is further advantageous in that it permits 10 avoidance of costly distillation procedures.

ABSTRACT OF THE DISCLOSURE DETAILED DESCRIPTION Styrene is Separated from Organic miX'fllYeS Comprising The process of the present invention comprises constyrene and ethylbenzene by contacting the mixture (feed tacting an organic feed mixture comprising styrene and mixture) against one side of a polyurethane elastomer ethylbenzene against one side of a polyurethane elastomer membrane under pervaporation permeation conditions permeation membrane and withdrawing at the second and withdrawing at the other side a vaporous mixture side a mixture having a higher concentration of the prefhaving increased styrene concentration. The polyurethane erentially permeable alkene than in the aforesaid feed elastomer employed contains polyether or polyester groupmixture. It is essential that the mixture at the second side ings. Examples of polyurethane elastomer polymers are be maintained at a lower chemical potential than the mixpolymers of the formula ture on the feed side. It is also essential that the product be Withdrawn at the second side in the vapor state. In the commercial utilization of the process multistage operation is feasible since this permits the operation of individual 0 0 rstages at various concentrations and temperatures in order 4 4 to achieve e optimum driving force for the process.

For each individual stage the efl ectiveness of the separa- 11 is an integer ill the range of 3-500 and m 15 all Integer tion is shown by the separation factor (S.F.). The separawherein R, R \R and R are divalent organic radicals, R is hydrogen or an organic radical, A is indicating the degree of polymerization. tion factor (S.F.) is defined as the ratio of the concentrations of two substances, A and B, to be separated, divided BACKGROUND OF THE INVENTION into the ratio of the concentrations of the corresponding substances in the permeate Field of the invention C C in ermeate This invention relates to a process for separating styrene S.F. MEL};

from organic mixtures containing same. In a particular (CA/CB) m pern 1eant aspect this invention relates to a process for the separation Where CA and CB are the Concentratlon 0f the Preferenof styrene from organic mixtures comprising styrene and s permeable component d y other p e t of ethylbenzene by preferential permeation through a polythe mlxtur? or the Sum of other components p e ymer membrane under pervaporation conditions. In a more r In carrymg ouhthe Process of the P e lHVeIlhOH, particular aspect this invention relates to a process for e fl feed slde 0f the a e 18 such that the the Separation f styrene f organic mixtures actlvrtles of the components are greater than the activiprising Styrene and ethylbenzene by contacting the said ties on the second side of the membrane. Preferably, the mixture under pervaporation permeation conditions f slde 15 above atlhosphel'lc P P and h ond against one side of a polyurethane elastomer membrane, slde below f h Pressure SD11 e preferably, said polyurethane elastomer containing groupings Selected the second side is malntalned such that the pressure diff the group consisting f polyester and polyether and ferentral is greater than 0.01 atmosphere. A further prerecovering on the other side a vaporous mixture rich in fetred mode of opfiratlon 1S Wlth the Second S de maln- Styrene. tamed at a v acuumof greater than 0.2 mm. Hg. 0

Description f the Prior art The term Chemical Potential 1s employed hereln as described by Olaf A. Hougen and K. M. Watson (Chemical Process Principles, Part II, John Wiley, New York, 1947). It is related to the escaping tendency of a substance from any particular phase. For an ideal vapor or Separation of monoalkenes such as styrene from organic mixtures such as mixtures of styrene and ethylbenzene has been accomplished by distillation procedures. Separation of azeotropic mixtures of organic materials such as mixgas, this escaping tendency is equal to the partial prestures of styrene and 2-chl0r0etha 0l and ur 0f sure so that it varies greatly with changes in the total ethylbenzene and 2-chloroethanol by membrane permeapressure. For a liquid, change in escaping tendency as a tion through certain polymer membranes followed by disfunction of total pressure is small. The escaping tendtillation is also known to the art, for example from US. ency always depends upon the temperature and concen- Pat. 2,953,502 issued Sept. 20, 1960 to R. C. Binning tration. In the present invention, the feed substance is d Rob t J Lee, gypically a liquid solution and the other side of the memrane is maintained such that a vapor phase exists. A SUMMARY OF THE INVENTION vapor feed may be employed when the mixture to be It has been discovered in accordnace with the present separated is available in that form from an industrial invention that styrene is effectively separated from orprocess or when heat economies are to be effected in ganic mixtures comprising styrene and ethylbenzene by multi-stage process. contacting the mixture under pervaporation permeation The feed side may be at pressures less than atmosconditions against one side of a polyurethane elastomer pheric, but preferably greater than atmospheric, and also at pressure over and above the vapor pressure of the liquid components. The collection or permeate vapor side of the membrane is preferably less than atmospheric pressure, but under proper feed side conditions, also may be greater than atmospheric pressure. The total pressure on the feed side is preferably between p.s.i. absolute and 5,000 p.s.i.g. The conditions are always such as to maintain a higher chemical potential on the feed side than on the collection or vapor side.

The temperatures on the feed side and the collection side may vary over a wide range. However, temperatures should be avoided which cause substantial decomposition of any of the organic materials in the mixture or of the membrane, and which cause the vapor pressure on the collection side of any of the permeated materials to be exceeded by the pressure being maintained on that side. Typically, an increase in temperature causes an increase in permeation rate.

In accordance with the present invention, separations are carried out for removal of the preferentially permeable styrene through the membrane with the said alkene in a higher concentration than in the feed being recovered from the collection side of the membrane as a vapor with the imposition of a state of lower chemical potential on such collection side of the membrane. For example, a mixture of styrene and ethylbenzene may be applied to one side of a flat diaphragm or membrane to accomplish removal of at least a part of the styrene leaving a more highly concentrated ethylbenzene solution on the feed side of the membrane. A state of lower chemical potential is maintained on the collection or downstream side of the membrane by vacuum e.g. from 0.1 mm. Hg to the vapor pressure of the organic component of the mixture which has the lowest vapor pressure at the membrane at the respective temperature as long as the vapor phase is present on the downstream side. In the system referred to above, the styrene selectively passes through the membrane with the styrene-rich composition being removed rapidly as vapor from the collection side of the membrane.

In contrast to the present invention the employment of permeates in liquid phase on the second side of the membrane is impractical because the applied pressure has been found to be prohibitively high e.g. up to 1,000 atmospheres being necessary because of osmotic pressure. Liquid-liquid permeation is largely an equilibrium phenomenon unless the osmotic forces are overcome while in contrast liquid-vapor or vapor-vapor separations are accordingly much more effectively carried out than liquidliquid separations.

The polymeric permeation membrane employed in the process of the present invention is a polyurethane elastomer membrane, the polyurethane elastomer having groupings selected from groups consisting of polyester and polyether. Such polyurethane elastomers are known to the art for example, from J. H. Saunders and K. C. Frisch Polyurethanes, Chemistry and Technology Part I. Chemistry, Interscience Publishers (1962). The polymers are typically of high molecular weight for example on the order of from about 10,000 to about 20 million and more.

Among the polymers suitable for the practice of the present invention are polyurethane elastomers having units of the formula L It with for example 4-20 carbon atoms (preferably aromatic), R is a divalent organic radical with for example 2-20 carbon atoms (preferably alkylene), R is hydrogen or an organic radical such as aromatic or alkyl (preferably lower alkyl), A is a member selected from the group consisting of and n is an integer of from 3 to 500. The units of the formula given above are recurring, m being typically in the range of from about 40 to about 20,000. When the units are recurring, the symbols in the various recurring units do not necessarily stand for the same thing in all the recurring units.

The polyurethane elastomers of the above formula and their preparation are known to the art, for example, from the article by S. B. Clough and N. S. Schneider, Journal Macromolecular Science, Physics B (4) p. 554 (December 1968). The polyurethane elastomers are prepared conveniently by capping a polyester or a polyether with a diisocyanate and then extending the polymer with a glycol. The polyester can be prepared, for example, from a diol (preferably an aliphatic diol containing 2-20 carbon atoms) such as butanediol, hexanediol, and dodecanediol and a dibasic acid (preferably an aliphatic dibasic acid containing 3-20 carbon atoms) such as adipic acid and sebacic acid. The polyether (preferably 2-12 carbon atoms) can be, for example, polybutylene oxide, polypropylene oxide and polyethylene oxide. The diisocyanates typically contain 4-20 carbon atoms. Examples include 2,4-tolylene diisocyanate, -35 tolylene diisocyanate, /20 tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate, tolidine diisocyanate, hexamethylene diisocyanate, m-xylene diisocyanate and dianisidine diisocyanate. The extending glycol typically contains 2-20 carbon (preferably alkylene glycols). Examples include butylene, propylene and ethylene glycol.

The membrane may be a simple disk or sheet of a membrane substance which is suitably mounted in a duct or pipe or mounted in a plate and frame filter press. Other forms of the membrane also may be employed such as hollow tubes and fibers through which or around which a feed is supplied or recirculated with the product being removed at the other side of the tube as a vapor. Various other shapes and sizes are readily adaptable to commercial installations. The membrane, of course, must be insoluble in the organic medium to be separated. Membrane insolubility as used herein is taken to include that the membrane material is not substantially solution-swellable or sufiiciently weakened by its presence in the solution to impart rubbery characteristics which can cause creep and rupture under the conditions of use including high pressures.

The art of membrane usage is well known with substantial literature being available on membrane support, fluid how and the like. The present invention is practiced with such conventional procedures and apparatus. The membrane, of course, must be sufliciently thin to permit permeation as desired, but sufficiently thick so as to not rupture under pressure conditions employed. Typically, suitable membranes have a thickness of from about /2 to about 25 mils.

The membrane may be prepared by any suitable procedure such as for example by casting a film or spinning a hollow fiber from a dope containing polymer and (preferably alkylene), R is a divalent organic radical 7 solvent. Such preparations are well known in the art.

6 The following examples illustrate specific embodiments polyester and polyether and withdrawing at the second of the present invention. In the invention, the membranes side of the membrane a vaporous mixture having a higher employed were in the form of film disks and were mounted concentration of styrene than the aforesaid feed mixture in a membrane holder. The membranes varied in thickwith the mixture at the second side being maintained at a ness from 5 to 20 mils. 5 lower chemical tpotential than the mixture on the other side of the mem rane. EXAMPLE 1 2. The process of claim 1 wherein the pressure on the Membrane permeations were conducted for the purpose second side of the membrane is less than atmospheric of separating styrene from an organic liquid consisting pressure and lower than the pressure on the other side of of 70 wt. percent styrene and 30 wt. percent ethylbenzene the membrane. using polyurethane elastomers containing polyester and 3. The process of claim 1 wherein the organic mixpolyether groupings. Separations were carried out under ture is a liquid mixture. pervaporation permeation conditions at approximately 4. The process of claim 1 wherein the membrane com- 22 C. In each run preferential permeation of styrene prises a high molecular weight polyurethane elastomer of was effected. In each run the pressure on the liquid side the formula I" O 0 0 lil "I 0l 0 l- O O 0 I -O( -IiI-R -H-N-iiO%A3O N-R Ni70-R 0- 1;I-RN-ii-OEA}O( !NR Ni0- L H a 1. EL 1 r l. was amtospheric and the pressure on the vapor side was where R is a divalent organic radical having from 2 to 0.1 mm. Hg. The results are shown in the table. 20 carbon atoms, R is a divalent organic radical having from 3 to 20 carbon atoms R is a divalent or anic LE 2 g EXAMP radical having from 4 to 20 carbon atoms, R is an organic The Procedure f Example followed to sapaliate divalent radical having from 2 to 20 carbon atoms, R Styrene q a mlxtlll'e vomprlslng styrene and y is a member selected from the group consisting of hydroe e e a urethane based on p ythe y gen and an organic radical having from 1 to 12 carbon (1101 sebacaie) pp Wlth 2,4-t01Y1ene dusocyanate and atoms, n is an integer of from 3 to 500 and A is selected extended w1th propylene glycol. from the group consisting of While the invention has been described with reference to particular embodiments thereof, it will be appreciated I: O and that modifications and variations are possible without deii-R'-- i R R Parting from the mvenuon' 5. The process of claim 41 wherein A is TABLE :-0H-0-omc]1 Rate X104 i i grams our/ Sepa- 4 R4 Run 11.3 omfl/riiil of ration No; Membrane membrane factor 6. The process of claim 4|- wherein A is 1.... Urethane based on poly(butanediol 3, 800 1.41 O O ad1pete 2.--": Ur glaape based on po1y(butylene 1,085 1. 23 3.-...:: U gtig n e based on poly(propylene 695 1.24 D 4...": uieghgiie based on po1y(ethylene 90 1.68 References Cted UNITED STATES PATENTS l Prepared by reaction with diphenylmethane diisocyanate followed 2 970 106 1/1961 Bi i et L 2 0 R by i f 3,228,876 1/1966 Mahon 260 674 R What 18 clalmed 3,226,934 4/1973 Strazik et al 260-669 A 1. A process of the separation of styrene from an organic feed mixture comprising styrene and ethylbenzene CURTIS DAVIS, Primary Examiner which comprises contacting the said feed mixture against one side of the membrane comprising high molecular us, L weight polyurethane elastomer, said polyurethane con- 260-674 R taining a grouping selected from the group consisting of 

